A unidirectional broadcasting system consists of a number of information providers which are connected and send information to a broadcaster that has the function of transmitting the information received from the information provider to a plurality of data receivers through a broadcasting channel, wherein data receivers have no possibility of sending any information back to the information providers or to the broadcaster. A broadcast system can reach a high number of data receivers in a large territory, theoretically without any limit. Examples of unidirectional broadcasting systems are television and radio broadcasting systems.
The availability of well-working broadcasting systems such as the television systems, able to allow the transmission of data to many end-users distributed in a large territory over a broadcast channel without considerable added costs, increases the research in such a field.
A method of providing unidirectional transmittal of data from a plurality of information providers to one or more data receivers over a broadcasting system is disclosed in the published European Patent Application EP-491069-A1. This application describes a connection-oriented transmission protocol wherein connection packets activate a "virtual" data channel, over which data can be conveyed from a calling terminal (information provider) to a called terminal (data receiver). The connection can be terminated by a disconnection packet or by using timeout mechanisms. Connection packets cause a calling terminal to be actually connected to a called terminal over the virtual data channel only if the destination address of the connection packet matches the specific calling terminal address, stored in the called terminal. This virtual data channel is identified only by the calling terminal address. Data packets are identified only by the destination address so that data flow can be routed over the corresponding virtual data channel. The information-providers act as sources of information consisting of a plurality of variable length messages and issue control packets, enabling/disabling packets and data packets. Each data receiver has a unique address as a permanent attribute. The enabling/disabling packets selectively enable or disable a specific data receiver or a specific group of data receivers. The grouping of the data receivers is managed and updated remotely by each information provider through control packets.
However, when the transmission system is unidirectional, like in a broadcasting channel, information is transmitted on an optimistic basis. In fact, through the broadcasting channel, the called terminal cannot inform back the calling terminal about errors occurring either over the channel or in the called terminal itself (e.g. terminal failure or temporary problems, unrecovered long duration noise, etc). Beside that, connection oriented protocols used over broadcasting channels are critical because of the intrinsic high dependency of data reception on the connection packet. The loss of this packet causes the loss of all subsequent data packets.
Consequently, it can be useful to retransmit the same information over the broadcasting channels, at different times. For example, the whole set of packets could be retransmitted again more times at time intervals in the optimistic expectation that the cause of the loss of the connection packet has ceased or has been removed and that this favourable condition lasts for a time period sufficient for the data receiver to successfully and consecutively receive the entire set of packets. Unfortunately, experience has shown that transmission disturbances or interruptions may last for long times and in some cases the above approach is unsuccessful. In fact the possibility of losing the connection packet upon long duration transmission disturbances or interruptions leads to the loss of the corresponding whole set of packets thus frustrating the expected benefits of the retransmission.
Therefore there is a need for a method and system allowing a data receiver to receive individual data packets or groups of data packets for later use in the re-establishment of the entire set of transmission packets.